Components located in certain sections of gas turbine engines, such as the turbine, combustor and augmentor, are often thermally insulated with a ceramic layer in order to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. These coatings, often referred to as thermal barrier coatings (TBC), must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles.
Coating systems capable of satisfying the above requirements typically include a metallic bond coat that adheres the thermal-insulating ceramic layer to the component. Metal oxides, such as zirconia (ZrO.sub.2) partially or fully stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or other oxides, have been widely employed as the material for the thermal-insulating ceramic layer. The ceramic layer is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD) which yields a strain-tolerant columnar grain structure. Bond coats are typically formed of an oxidation-resistant diffusion coating such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant alloy such as MCrAlY (where M is iron, cobalt and/or nickel). Aluminide coatings are distinguished from MCrAlY coatings, in that the former are intermetallics while the latter are metallic solid solutions.
While coating systems of the type described above are widely employed, the requirement that the coating system remain adherent throughout many heating and cooling cycles is particularly demanding because the coefficient of thermal expansion (CTE) of ceramic materials is significantly lower than those of the superalloys typically used to form turbine engine components. Such differences in CTE, in combination with oxidation of the underlying bond coat or substrate, eventually lead to spallation of the coating system.
An additional desired characteristic for a coating system of a gas turbine engine component is for the outermost surface of the coating system to be extremely smooth in order to promote the aerodynamics of the component surface. While relatively smooth ceramic coatings can be produced with spray methods such as those noted above, particularly with PVD techniques, smoother surface finishes would be desirable. In general, deposition techniques noted above tend to produce ceramic coatings that are relatively porous, which is advantageous in terms of achieving a low coefficient of thermal conduction. However, porosity promotes surface roughness--ceramic coatings deposited by PVD generally have surface roughnesses of about 60 .mu.inch (about 1.5 .mu.m) R.sub.a and higher, and those deposited by APS and LPPS typically have surface roughnesses of about 260 to 400 .mu.inch (about 6.6 to 10.2 .mu.m) R.sub.a. Ceramic coatings deposited by conventional spray methods on components with complex geometries are further prone to such surface flaws as shadowing effects (thin or beaded regions caused by partial masking due to part shape) and slumping (thicker regions formed by movement of material to low portions of a part due to gravity).
In view of the above, it can be appreciated that there is an ongoing demand for gas turbine engine components that can be produced with adherent thermal barrier coatings having exterior surfaces that are denser and smoother for improved aerodynamical performance.